京都大学防災研究所年報 第 65 号 A
DPRI Annuals, No. 65 A, 2022

Friction Level of Faults During Large Earthquakes
James MORI(1)(1) Earthquake Hazards Division, Disaster Prevention Research Institute, Kyoto University
Synopsis
Temperature measurements were used to estimate the level of fault friction of large
earthquakes. These observations were made in boreholes drilled into fault-zones
following large earthquakes in Taiwan, China and Japan. From the temperatures, we areable to estimate the heat generated at the time of the earthquake and thus the level offriction on the fault. The results for all three earthquakes show that the coefficient offriction during the earthquake was 0.1 or less. This is much smaller than values for rockfriction measured in the laboratory. The friction determined from rock mechanics
experiments represents static friction values, while the temperature measurement values
are the dynamic friction during large slip of earthquakes. So static friction of rocks ismuch higher than the dynamic friction during large earthquake slip.
Keywords: Earthquake, Fault, Friction, Temperature, Chi-Chi, Wenchuan, Tohoku-oki

1.

Research Career

First is a summary of my research career. I
received a PhD degree in geophysics at Columbia
Univ. in 1984 and then held three positions:
Seismologist, Rabaul Volcano Observatory, Papua
New Guinea (1984-1988), Research Scientist, US
Geological Survey in Pasadena, California (19841999) and Prof., Disaster Prevention ResearchInstitute, Kyoto Univ. (1999-2022). I lived 3 years
in Papua New Guinea, 11 years in the US and 23
years in Japan, however, in terms of research ideas,
first author papers and remembered experiences, thedurations of the three periods seem similar in mymind. In geophysics sometimes ‘apparent time’ isused, which measures time by counting significant
events. The apparent time for my three positions
seems about the same despite the large differences in
the actual time (Fig. 1). The total numbers of
published papers is more closely related to the actualtime, since that depends more on daily activities andcollaborations with other researchers.

Fig. 1. Summary of my three career positions. Topcolored line is actual time. Bottom line is ‘apparent’time.
1.1

Rabaul Volcano Obs. 1984-1988
My first job at the Volcano Observatory of thePapua New Guinea Geological Survey was an
exciting time. I observed eruptions at 4 different
volcanoes and felt the shaking from 4 M7earthquakes. Also there was very strong seismic and― 20 ―

Fig. 2. Some photos of eruptions in Papua New Guinea. 1985 Langila (left). 1985 Uluwan (center). 1986
Manam (right).
ground deformation activity in Rabaul caldera. No
eruption occurred in Rabaul at that time, but a largeeruption later in 1994 destroyed the town.
1.2 US Geological Survey 1988-1999
I returned to the US in 1988 and worked for 11years at the US Geological Survey (USGS) on theCaltech campus in Pasadena, California. Providing
public information about earthquakes to the manypeople in the Los Angeles region was very different
from sparsely populated Papua Guinea. Even asmall earthquake that was felt by people was a TV
news story. I was interviewed over 100 times on TV
and radio about earthquakes while working at theUSGS. Two large earthquakes, 1992 Landersearthquake (Mw7.3) and 1994 Northridge
earthquake (Mw6.7) were important events. The twoearthquakes caused considerable damage and werethe focus of much research. There were considerable
efforts to use the USGS research results to help
recovery and provide seismic hazard information forthe public. In 1992, I was a member of the Volcano
Crisis Assistance Team that monitored and provided
information on the large eruption of Mt. Pinatubo inthe Philippines.

Fig. 3. Caltech-USGS press room is busy after anearthquake in Los Angeles.

1.3 DPRI Kyoto University
In 1999 I came to Japan to become a professor at
the Disaster Prevention Research Institute (DPRI) ofKyoto University. Working at DPRI, teachingclasses in Japanese and English and living in Japan,
were all new experiences for me.
Soon after arriving in Japan, the Chi-Chi
earthquake (Mw7.7) occurred in Taiwan. With DPRI
colleagues we went to investigate the large surface
faulting and damage caused by the earthquake.
This was the beginning of my interest in thisearthquake and borehole drilling into faults to maketemperature measurements for estimating fault
friction.
2.

Fault-zone drilling and Measurements ofFault Friction

Estimating and understanding the level offriction during the faulting of large earthquakes has
been an active topic of research for decades inseismology. There have been many debates about
whether the level of fault stress is high (~100 MPa)or low (~1 MPa) during earthquakes. In laboratory
experiments the coefficient of friction for rocks has
been measure in the range of 0.6 to 0.7 (Byerlee,1978) which would indicate high stress. If thefriction is this high, large amount of heat would be
produced during earthquakes and the slip during
earthquakes would produce temperatures highenough to melt the rocks. However, for the San
Andreas fault, the measured values of heat flow are
relatively low, much lower than what would beexpected for a coefficient of friction of 0.6 to 0.7.

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Also melted rocks (pseudotachylytes) are very rare
along the fault. This is called the San Andreas Heat
Paradox (e.g. Lachenbruch and Sass, 1980). There
have been discussions about whether or not thelaboratory friction values are representative of
natural faults, and if the heat flow measurements
accurately reflect the frictional heat on the fault.
We realized that one of the most direct ways tostudy this problem was to measure the temperaturechange across a fault soon after a large earthquake.
If you rub your hands together on a cold day, they
get warmer because of the frictional heat. In thesame way, a fault gets hotter from frictional heat
when the two sides of a fault slide in opposite
directions during an earthquake. From the observed
temperature difference, the amount of generated heat,and thus the friction can be calculated. Working withcolleagues at DPRI, one of my main research longterm objectives was to estimate the friction level bymaking fault-zone temperature measurements soonafter large earthquakes.
There are several requirements needed fortemperature measurements to estimate the friction.
1) The measurement needs to be done soon after theearthquake, preferably within about a year. 2) The
fault slip where the temperature measurements aredone has to large (several meters). 3) A boreholemeasurement is necessary because the fault is
usually complicated at the surface and thedisplacement is usually not seismic slip (negligiblestress drop). This means that the observed large fault
slip needs to come to shallow depth that is accessible
with a borehole (within 1 or 2 km).
2.1 1999 Chi-Chi, Taiwan Earthquake
The first chance to make fault-zone temperature
measurements was following the 1999 Chi-Chi,
Taiwan earthquake (Mw7.7) which produced large
surface faulting over most of the 70 km length of theChelungpu fault. The Taiwan Chelungpu-Fault
Drilling Project (TCDP) was started to investigate
the properties of the fault with boreholes into thefault-zone. The DPRI contribution was to makeborehole temperature measurements across the fault
zone to estimate the fault friction. Establishing theproject and drilling the main borehole took time, sothat the temperature measurements were done in2005, which was 6 years after the earthquake. This
was quite late and the temperature had mostlydissipated, but we still observed a small anomaly of
about 0.06 oC across the fault in a region where theslip was 5 meters.

Fig. 4. Temperature instrument used in Taiwan
At this time there were no commerciallyavailable thermometers that could be used, so wecontracted a company to build an instrument that had
two quartz temperature sensors and data loggers.
The instrument was about 6 meters long and quite
heavy (Fig. 4). For this analysis, we used the residual
temperature, which is the measured temperatureminus the linear temperature gradient with depth.
The residual temperature shows a small peak
centered on the fault (solid line in Fig. 5). Fig. 5
also shows calculated values for the residual
temperature assuming various values for thecoefficient of friction (thin lines). The data are best
fit with a coefficient of friction of 0.08. This is much
lower than the laboratory values of 0.6 to 0.7
mentioned in section 2. This result was the firstreported friction estimate from temperature
measurements following an earthquake (Kano et al.,2006). However, the noise level in the data isrelatively high and the measurement was done 6years after the earthquake, so the reliability of theresult might be questioned.

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Fig. 5. Results of temperature measurement for Chi-Chi, Taiwan earthquake (thick black line) and model
calculations for coefficients of friction values (thin lines) for 0.06, 0.08 and 0.10. From Kano et al., 2006.

2.2

2008 Wenchuan, China Earthquake
The second chance to make temperature
measurements came following the 2008 Wenchuan,
China earthquake (Mw7.9). This large earthquake
produced surface faulting at many places along theLongmenshan Fault zone. The Wenchuan Fault
Scientific Drilling project (WFSD) was establishedby the Institute of Geology, Chinese Academy ofGeological Sciences, to drill a series of boreholes atseveral sites. We carried out temperature
measurements in Hole 2 starting 10 months after theearthquake and continuing for 5 years. The borehole
reached the fault at a depth of 595 meters. By thistime there were commercially available temperature
sensors that were much smaller and easier to use
compared to the instrument we built for the Taiwan
measurements.
The results from Li et al. (2015) in Fig. 6 show asmall residual temperature anomaly of about 0.02 oC
across the fault where the displacement was 7 meters.
The temperature signal has a very small amplitude
and the time dependence of the shape was not quite
what we would expect for an anomaly caused by thefrictional heat. Using the very small amplitude of thesignal would give a coefficient of friction of muchless than 0.1. Because of the unusual characteristicof the temperature anomaly, it is questionable if thisis due to the fault friction. However, It can beconcluded that the fault friction is not as large 0.6 to
0.7 because, in that case there would be a large

anomaly that should be easily observed by ourtemperature measurements.

Fig. 6. Residual temperature for Wenchuan, China
earthquake. From Li et al., 2015.
2.3 2011 Tohoku-oki Earthquake
The 2011 Tohoku-Chihou Taiheiyou-oki
earthquake (Tohoku-oki earthquake) was the largest(Mw9.1) earthquake in Japan’s recorded history anda very important event in terms of seismological andtsunami research. Since the faulting near the Japan
Trench came to shallow depth (less than 1 km), thiswas a prime location for another fault-zone drilling
project to the friction level for a very large
earthquake.
The Japan Trench Fast Drilling Project (JFAST)(Mori et al., 2014) was a large multi-country effort
by the International Ocean Drilling Program (IODP)― 23 ―

designed to study the shallow portion of the fault that
had huge slip during the earthquake. It is importantto understand this shallow slip because it is the main
source of the large tsunami that devasted much of theTohoku coastal region.
The research ship Chikyu was used to drill
boreholes from the seafloor to the nearly horizontal
fault. The depth of the fault from the seafloor was815 meters. There were difficult technical problems
because the water depth was nearly 7000 meters.
After several borehole was drilled and a sample offault retrieved, temperature sensors were place in
one of the boreholes. The instruments continuously
measured the temperature from August 2012 to April
2013.
Fig. 7 (from Fulton et al., 2013) shows the resultsof temperature measurements that were recorded
during the time period from August to December
2012. Of the three fault zone temperature
measurements described in this paper, the Tohokuoki data are by far the best quality observations. The
measurements were made relatively soon after theearthquake (17 to 21 months) and the slip of the fault
was very large (50 m). The results of the residual
temperatures show a clear anomaly across the fault
with an amplitude of about 0.3 oC. The one-sided
width of the anomaly is about 5 meters whichmatches the theoretically calculations. The inferredcoefficient of friction from the modeling is 0.1.

3.

Summary and Conclusions

Table
1summarizes
thetemperature
measurements following large earthquakes for thethree fault zones in Taiwan, China and Tohoku
region. All three measurements show quite low
coefficients of friction of about 0.1 or less. This
result is quite different from the much higher valuesof rock friction that were obtained in laboratory
experiments. It is rather surprising that the fault
rocks can slip with such low friction. For example,Fig. 8 lists some values of coefficient of friction forvarious materials. Our results show that the frictionlevel is between rubber on wet concrete (car tire ona rainy day) and steel on ice (ice skating).
One important difference between the rockmechanics experiments and our temperature results,
is that the laboratory results are for small
displacements and reflect values similar to the staticfriction. In contrast, the temperature measurements
reflect the heat during the time of the large slip ofthe earthquake, so they represent the dynamicfriction. The static friction of the fault is likely quite
high and close to the laboratory values of friction,
however, once the slip begins on the fault, thefriction drops to a lower value of dynamic friction.
The drop of friction during slip of a large earthquake(slip-weakening) has been previously proposed andmay be due to thermal pressurization (Rice, 2006) orother mechanisms, such as mechanical lubrication or
flash heating. This difference between static anddynamic friction can explain the San Andreas Fault
Heat Paradox. The static friction of the fault zone
rocks is relatively high when the fault is not slipping
but slip-weakening mechanisms cause the drop ofdynamic friction to lower values when the large slipoccurs. The heat flow values reflect the dynamicfriction levels.

Fig. 7. Left: Observed residual temperature for theTohoku-oki earthquake as a function of depth(vertical) and time (horizontal). Right: Calculated
temperatures assuming a coefficient of friction of 0.1.
From Fulton et al., 2013.

Fig. 8. Coefficient of friction for various materials.

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Table 1. Summary of fault-zone temperature measurements.

Acknowledgements
The author thanks all the research colleagues,
students and supporting staff at DPRI, without whom
all of this research would not have been possible.

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― 25 ―(Received August 29, 2022)